The onion (Allium cepa) is one of the most popular plants in home vegetable gardens and it’s not particularly difficult to grow. However, knowing when and how to harvest onions can certainly pose problems, especially for novice gardeners.
Here are some tips on how to get it right!
One Plant: Three Seasons
Few gardeners realize that you can, in fact, harvest the onions in almost any time during the growing season.
If you harvest onions when they’re very young, before a real bulb has formed, you’ll get what are known as green onions or scallions. There are actual cultivars of A. cepa grown specifically for use as green onions, as they don’t form much of a bulb, but any onion can be used as a green onion. With green onions, not only can the stem be used, but the leaves too are edible. They can only be stored for short periods, though: a week or so.
You can also harvest onions when the bulb is well-formed, but not yet at its full size, normally in June, July or early August. These young onions may be called summer onions, salad onions or pickler onions. Some people call them pearl onions, but the actual pearl onion belongs to a different species, A. ampeloprasum sectivum.
Summer onion bulbs are used fresh and can also be pickled, but otherwise don’t store well. They won’t last much more than a week in the refrigerator. Don’t forget the leaves are still fresh and perfectly edible at this stage.
The third stage of harvesting is the classic one for harvesting: when the bulb is at full maturity. This is the storage onion, so-called because it can generally be stored for months, although some cultivars are better storers than others. In general, red and yellow onions store longer than white ones.
Depending on the variety and the climate, onion bulbs can reach full maturity as early as June. That would be the case with short-day or day-neutral onions grown in mild climate regions. Northern gardeners will likely be growing long-day onions, and they only start to form a bulb after day lengths reach 14 to 16 hours at the summer solstice, so most won’t be ready until August. In my area on the 47th parallel, they generally mature between mid-August and early September.
What is interesting about storage onions is that they let you know when they’re ready for harvest. The foliage begins to die back and fall over, a sign they are fully mature. If the bulbs in one row don’t all ripen at the same time, some gardeners push over the leaves of recalcitrant plants with their foot to force their maturation a bit, but if you wait just a few days, they’ll join the others all on their own.
These full-size bulbs can be eaten fresh, but if you want to store them, you have to dry them out first, a step called curing.
To do this, first dig up the bulbs. It’s best to use a spading fork rather than a shovel, as there is less risk of slicing the bulbs by accident. Do this on a dry day when no rain is expected for a few days.
Shake the bulbs to knock off most of the soil (but don’t rinse them: you don’t want them exposed to moisture at this stage!). You can then simply leave the bulbs and = foliage lying on the ground to dry in the sun for two or three days. If there is a risk of rain or frost, though, carry out this step in curing under shelter: in a tool shed, a garage or on a covered porch, for example.
Avoid injuring the bulbs as you harvest. Even just bruising them leads to a risk of decay further down the road. Use any accidentally damaged bulb promptly before rot can set it.
After the first few days of drying, the bulb will already be developing a thicker envelope and will be less fragile. Now is the time to clean the bulbs. Just do so roughly, using your hands or a soft brush to knock off soil particles. Never wash them: you want them dry, not moist. Don’t worry if the onion skin comes free as you clean: that’s perfectly normal and it will soon grow a replacement. However, do remove any rotting or damaged bulbs.
You have two choices at this point. Storing the bulbs braided or dry them further in racks.
If you’re going to braid them, do so right away, while the leaves are still fairly green and pliable, then suspend the bulbs from the ceiling in your storage area.
If you’re going to be using racks, spread the bulbs, foliage and all, on some sort of perforated support (wooden onion drying racks were once common, but most gardeners these days use ventilated plastic or metal shelving). Two or three weeks later, when the foliage is thoroughly dry, just pull it off or cut remove it by cutting it back to just about the bulb. Cut off the roots as well. Your bulbs will be, at this point perfectly cured.
For onion bulbs to keep a long time, you’ll need to supply cool temperatures, low light, dry air … and good aeration. That’s why commercial onions are usually sold in mesh bags rather than sealed in plastic like some other vegetables.
You can store onions on racks, in wooden crates or in perforated cardboard boxes, preferably in a spot where temperatures will remain cool: 40 to 46˚ F (4 to 8˚ C): basement or garage, for example. Ideally, the bulbs shouldn’t touch, as that will allow better air circulation, but you can also arrange them in layers. If so, it would be wise to check them every month, removing any rotting bulbs.
Most good storage onions will keep for six months, some even up to a year if they are carefully cured and stored under ideal growing conditions.
Enjoy your harvest!